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Bovine Spongiform Encephalopathy (BSE) ›
Bovine Spongiform Encephalopathy (BSE)
Author: Prof Gavin R Thomson
Licensed under a Creative Commons Attribution license.
PATHOGENESIS
There is experimental evidence that after oral infection of cattle infectivity is first detectable after about 6
months in the distal ileum (probably associated with the Peyer’s patches) which reaches maximum levels
about 14 months after infection, i.e. 6 months before similar levels of infectivity arise in the central
nervous system (CNS). By 36 months after exposure levels of infectivity in the ileum were found to have
declined (Arnold et al., 2009). Traces of infectivity have also been reported in the palatine tonsils of cattle
killed 10 months after experimental exposure (Wells et al., 2005).
The route by which infectivity reaches the CNS was shown, also experimentally, to occur by anterograde
movement of PrPSc along two pathways of the autonomic nervous system from the gastro-intestinal tract
(GIT), i.e. via: (1) the coeliac and mesenteric ganglion complex, splanchnic nerves and lumbar/caudal
thoracic spinal cord (i.e. sympathetic innervation of the GIT) and (2) the vagus nerve (parasympathetic
GIT innervation ‒ Hoffmann et al., 2007). Spread of infectivity to the dorsal route ganglia (DRG) appears
to occur at about the same time or after infection of the CNS; the levels of infectivity in DRG being in the
order of 10-fold lower than in the CNS. In the CNS infectivity levels are variable – 102.4-105.2 mouse i.c. or
i.p. D50/g1 (Arnold et al., 2009). Spread to the peripheral nervous system and adrenal glands has also
been identified in experimentally infected cattle; this occurred at the time or shortly after PrPSc appeared in
the CNS (Masujin et al., 2007).
In cattle in the incubation phase of the disease the doubling-time of PrPSc in the brain was found to be
about 1.2 months and infection of the rostral medulla oblongata preceded infection of the lumbar/thoracic
spinal cord while infection of that part of the spinal cord preceded infection of the cervical spinal cord
(Arnold et al., 2009). In diseased cattle the brain, followed by the spinal cord contain by far the most
infectivity.
In general all TSEs induce similar changes in the CNS consisting of a combination of predominantly
extracellular deposition of PrPSc ‒ the intensity, pattern and distribution of deposition being highly variable
‒ in the form of amyloid (rich in β-sheets) produced by neurons, and spongiform change with neuronal
vacuolation and degeneration accompanied by astrocyte and microglial reaction. There is
characteristically no inflammatory reaction; consistent with PrPSc being recognized immunologically as
1
Infectivity measured by intra-cerebral or intra-peritoneal inoculation of mice with dilutions of brain
material suspensions – the dose required to induce the disease in 50% of the mice being assessed as
representing one infectious dose
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Livestock Health, Management and Production › High Impact Diseases › Contagious Diseases › .
Bovine Spongiform Encephalopathy (BSE) ›
‘self’. These changes presumably result in disturbances in neurological impulse transmission, the upshot
of which is the clinical manifestations associated with TSEs.
The distribution and quantities of infectivity in infected cattle are important in determining the risks that
BSE poses to people who consume foods containing bovine tissues. This aspect has therefore been
carefully studied. Despite some reports to the contrary, there is little evidence that organs, tissues or
secretions other than those mentioned above contain significant amounts of PrPSc in infected animals or
those incubating the disease (Arnold et al., 2009). This includes milk, semen, embryos, pure striated
muscle (i.e. beef from which bones and lymph nodes have been removed) and fat (tallow – Terrestrial
Animal Health Code, Chapter 11.5; OIE, 2011). Therefore the two major food sources provided by cattle
(milk and beef) are safe, or can be made so, for consumers as far as BSE/vCJD are concerned.
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